Like Pavlov's dog was trained to salivate when a bell was rung, so sharks know to expect food when motorboat engines hum overhead. This is according to marine biology researcher Matt Dickson at the KwaZulu-Natal Sharks Board, who said association of noise from boats and the promise of "chum" being thrown overboard caused sharks to gather.He was reacting to a recent study at the University of Exeter which found that sharks were given a competitive advantage over prey because the sound of motorboat engines "increased stress levels" in fish and reduced their ability to flee, halving their chances of survival.The study was led by researchers at the University of Exeter, with assistance form scientists at the University of Bristol and others based in Australia and Canada.Exeter academic Stephen Simpson said: "We found that when real boats were motoring near to young damselfish in open water, they became stressed and were six times less likely to startle to simulated predator attacks compared to fish tested without boats nearby."Dickson said while he had not studied whether or not motorboat sounds were giving predators a competitive advantage, he had noticed that fish behaviour changed when boats were around."The closest thing to this that I have noticed is that when diving at Aliwal Shoal off Durban. Sharks have learnt that they will be fed by dive boats chumming for sharks."So, the engines are not causing a competitive advantage that results in a better chance of predation," Dickson said...